99 Lumina overheating

I was driving my car to my friend's house today and it seemed as if the car was taking a little longer to get warm than it usually does. I got there and the heater wasn't working anymore. I left it in his driveway to warm up to see if the heater would work. I went outside and the heater didn't work and the temp gauge was at half. I turned off the car, let it cool down and tried to make it back to my apt since I figured something was wrong. I started the car drove it about a half mile and the temp hand started to rise. It didn't hit the red, but it was about to it. I took it back to his driveway. He told me that he noticed that some fluid had leaked out of the car. About a cup or so (measuring cup size). I went to work and came back eight hours later and there was no water on the ground, no water in the overflow jug and I don't believe there was any in the radiator, but it was dark and I couldn't see, but I couldn't feel any when I stuck my finger down in it. Here's some other things you may need to know about my car. I used a stop leak thing about 6 months ago for the intake gasket leaking. It was a very small leak, but since I used the stuff it sealed it and I haven't had a problem with it. Also, when I parked the car, it appears as if a very small amount of steam came out from under the hood, but it wasn't smoking. Also, where I live this is the first time that the temp had dropped below freezing for about two days straight that I didn't drive my car and if it ever is missing fluid from the radiator I would just add more water. So, could it have too much water, not enough antifreeze and I caused my heads to crack?

Also, motor 3.8. Should I sell it or is it worth fixing. I know that heating problems can be a bitch to fix as well.

Fill it will coolant mixture ASAP and start looking for leaks.

Also, motor 3.8. Should I sell it or is it worth fixing. I know that heating problems can be a bitch to fix as well.
if you live in a region that does not get freezing temps you must add at least 50% antifreeze coolant and make sure it is the proper type,freezing , lubrication and corrosion are problems that antifreeze controls obviously water is much cheaper but in the long run you will save alot of cash by maintaining the proper level mixture of your cars coolant system....in your case you probably cracked something and all the coolant ran out.put some moonshine it that engine then it won't freeze.

It sounds to me like the coolant has found it's way into the oil via a gasket leak (lower intake, perhaps). Check the dipstick for brown stuff that is a mixture of oil and coolant. This happened to me recently, very similar symptoms.